Radio navigation beacon



Dec. 1, 1959 l.. A. DE RosA Erm. l 2,915,750

RADIO NAVIGATION BEACON Filed July 12, 1957 2 Sheets-Sheet l1 l i l l -Y 27 o )IX @sca/Vf@ V5 Inventors v O/.f A. e l

Dec.,.1,y 1959 L. A. DE RosA ErAl. y 2,915,750

v RADIO NAVIGATION BEACON Filed July 11?., 1957 2 Sheets-Sheet 2 gay. 6

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RADIO NAVIGATION BEACON Louis A. De Rosa, Upper Montclair, Martin `Masonson,

'Nutley, and Frank I. Lundberg, Newark, NJ., assignors to International Telephone and Telegraph Corporation, Nutley, NJ., a corporation of Maryiand Application July 12, 1957, Serial No. 671,435

4 Claims. (Cl. 343-106) This invention relates to radio navigation beacons and diierence between the carrier directly radiat^d and they one -reected to a receiving point intiuences the size of [error in an estimation of the azimuth, just as the phase ydiierencel in the modulations on .these carriers. The obstacle generates a ghost, the reflected signal of the desired transmitted signal, whose radio frequency phase relative to the desired signal could be anything. It could be such as to produce a large azimuth error at the receiver, or a small one, and even zero azimuthal'A error. Whatever it is, it is a certain error depending on the given relative displacement of transmitter, receiver yand obstacle. t

But suppose the transmitter position changes somewhat. It does not have to move far to make a radical change in the electrical and hence effective displacements of the transmitter, receiver and obstacle. The azimuthal error would now, generally, be different from its previous value, that is, before the change in position. If it were large before, it may now be small or anything else. Now consider varying the transmitter position locally about some point. With each .configuration of transmitter, obstacle, and. receiver, a certain azimuthal error is incurred. An average estimate of azimuth can be found. vThe azimuth so determined will, in general, have a smaller variance, which is to say, will be more accurate. One embodiment of this principle would be to have a multiplicity of identical antennas spaced in some definite arrangement, and switch from one antenna to another in some definite sequence. would have to know the transmitting program to separate out the individual contributions from each transmitter. This is, however, too complicated and expensive for practical use. Actually, we provide the receiver with a continuous stream of azimuthal data, which at each instant (as against discrete changes in transmission) is differently corrupted by the ghost signal. The receiver operates on this data and produces an average estimate of azimuth. However, care must be taken with the -transmitted signal in order not to introduce error .that

would not occur were the antenna stationary. l

' yIt is therefore an object of this invention to provide a radio navigation beacon wherein the ,transmitting antenna or the radiation therefrom is continuously vared yin position or character at the same time without requiring any complicated modification of conventional receivers.

It 'is a further object to `provide a radio navigation beacon having a directional rotating antenna wherein Of course, the receiver' arent tion.

Aradio frequency carrier is obtained by rotating the antenna about an axis displaced from the vertical axis of `the, antenna.

The above-mentioned and other features and objects of this invention Vwillbecorne more apparent by reference to the 'following description taken innconjunction with the accompanying dra-wings, vin which:

Fig. l is one embodiment of this invention;

Fig. 2 is a view along line 2-2 of Fig. l;

Fig. 3 is the receiving system for use with the trans- -mitter of Fig. l;

Fig. 4 is a View illustrating the space relations between the transmitting antenna, receiver and an obstacle;

Fig. 5 is a vgraph of the reflection error'versus the angle between the lines joining transmitter Ato receiver and transmitter to obstacle; and

Fig. 16 is another embodiment of this invention. Referring now to Figs. l and 2, there is shown an omni-directional radio navigation beacon y1 consisting yof a transmitting antenna 2, such as is used in the VOR navigation system, a transmitter 3 `coupled to the antenna -1 by Lleads 4 and 5, and a motor 6 coupled to the antenna 1 by means of shaft 7 and a boom 8. The antenna 1 includes a small rotatable dipole 9, driven by a motor 10 which is supported on a dielectric platform 11. The dipole is rotated to produce a rotating figure of 8 radia- Above the dipole 9 is a loop antenna 12 adapted to Vproduce a circular radiation. The platform 11 and ,the yloop antenna 12Vare supported by rods 13 coupled to Va base supportV 14 to which the boom'8 is attached. The

loop antenna 12 is in the form of a slotted disk which may be adjusted vertically on the rods 13 to provide the desired resonance loading for the dipole 12 and thereby compensate for the capacitive reactance thereof and thus obtain radiation eiiiciency.

For a more complete understanding of the 'structural details of the loop antenna 12, reference should be made to Fig. 2. The conductive disk 15 is circular and made up of copper, brass, aluminum or other conductive -material. The disk is provided with four radial slots, l16, 16a, 17 and 18, disposed at right angles thereby dividing the disk into four equal sectors. vAny number of slots, however, could be employed, the number being determined to insure a constant current distribution for a given `diameter disk. These slots may not be straight and radial but may be of other configurations so long as all of the slots contained in the antenna disk are of substantially the same configuration. The loop antenna is fed by applying a potential across the four slots at the peripheral portion of the disk.y The coaxial line 19 is brought to the center of the disk to a junction box 20, whereby the feed is divided betwen the four slots, eachslot being fed by a coaxial line 21 extending from the box 20 to the outer end of the slot where the inner conductor 22 is exposed crosswise of the slot. A second length of coaxial line 23 extends beyond the exposed center conductor 22 and constitutes an open end coaxial stub for matching purposes. It will thus be apparent that each of the four slots is fed at the outer end thereof, and that the feeding thereof is cophasal. The electrical length of the slots 16, 16a, 17,

. 3 i and 18 is determined by shorting bars such as indicated at 24, the shorting bars being adjustable so that a substantially perfect circular radiation can be obtained. The coaxial lines 21 and 23 for `each slot may be varied in vlength to satisfy the impedance at the junction box 20.

The rotating dipole antenna 9 is driven clockwise by the motor 10 at a speed in the VOR system of 30 cycles per second, which amplitude modulates the radio frequency carrier signal generated by the transmitter 3. The antenna 2 is simultaneously rotated counterclockwise about the axis of the motor 6 at a speed less than 30 cycles per second determined by mechanical reasons, such as the weight of the transmitting antenna and the length of the boom 8. The carrier signal passing through leads 4 and 5 is transferred to the loop antenna 12 and the dipole antenna 9 by means of slip rings 25 and 26. The receiving system 27, shown in Fig. 3, comprises a conventional receiver 28 coupled to a receiving antenna 29. The receiver 28 is coupled to a iilter 30 and the output of the lter 30 is fed into a utilization circuit 31.

The operation of the beacon can be explained more thoroughly with reference to Figs. 4 and 5. A rotating cardioid 32 is generated by the rotating dpolelantenna 9 and the xed loop antenna 12. Obstacle 33 reflects some part of the transmitted signal striking thereon to the receiver 27a. ing the receiver 27a and the obstacle 33 to the antenna `pattern 32 is designated by the symbol p. The rate of rotation of the antenna about its own axis clockwise is the rate of rotation of the antenna about an axis o at a distances from the antenna is a and opposite in direction to First let us consider the case where a is not present and the dipole rotates only about its own axis. In the ordinary VOR or TACAN (fundamental or course bearing indication) systems, the azimuth error has a maximum value e given by e=ak sin ga radians imum valve of e occurs at 90 degrees and 270 degrees and the minimum value occurs at 0 degree, 180 degrees and 360 degrees. Now let us consider the case wherein addition to the rotation rate of the antenna about its own axis, it is also rotating about the axis o at the distance s from the antenna at a rate a. The difference rate will be the eiective rate of amplitude modulation as the receiver sees it. Let that rate be p. The rate p is so chosen that the receivers used in VOR and TACAN can use the system described here without any change except for the addition of filter 30. As heretofore explained, the rate chosen for a depends on practical and mechanical factors. It may be one revolution per second or even tive revolutions per second as may be feasible. is then adjusted to give the correct modulation rate p. In the case of VOR p corresponds to 30 cycles per second. In this case we have a maximum error e' which is where ls== the radius of the circular path and A equals the radio.

frequency Wave length. As a specific example, consider the case of VOR with a reflecting obstacle at 1000 feet from the transmitter, For VOR X equals 10 feet. Taking s=8 feet, then The angle subtended by the lines join- For this condition of eccentric rotation plus rotation of the antenna about its axis, the maximum error is approximately 1A; the maximum error with a stationary antenna, which maximum e occurs at about p=15 degrees and p=345 degrees, as compared to 90 degrees and 270 degrees for the stationary antenna. However, in the case of the moving antenna, the receiver video signal beyond the second detector will consist of many harmonics of the rate frequency a and harmonics of the rate p. The desired information bearing signal is the one at frequency p. To extract it, it may be necessary to use a fairly selective lter 30 centered at p'.

Of course, the rotation of a 100 rnc. VOR structure at an 8 foot radius may in many situations not prove feasible. At higher frequencies, all dimensions become proportion- Iately scaled down leading to quite manageable structures. For example, the same error reduction noted above occurs in TACAN (fundamental or course bearing indication) at a radius of 0.8 feet, since TACAN` wave lengths are about l foot. t

It should be understood that the movement of the transmitting antenna 2 along a circular path is not the essential factor reducing the error, but rather it is `the variability in the phase differencebetween transmitted and reflected signals at the receiver. A similar reduction in error can be achieved by a direct phase modulation of the radio frequency carrier, instead of locally varying the antenna position. This embodiment is shown in Fig. 6, where ythere is illustrated a transmitting antenna 2a, identical to the antenna 2 of Fig. l, coupled to a transmitter 3a. The phase modulation of the radio frequency carrier is effected by phase modulator 36 coupled to the transmitter 3a. In `this embodiment the maximum error would be Here h=maximum deviation of carrier frequency expressed as a fraction of center frequency, and r is the distance to the reliecting obstacle. Clearly if is about the same as invention in connection with specific apparatus, it is to l energy of a radio frequency carrier to said radiating means, rst means associated with said radiating means to effect rotation of said pattern about the axis of said radiating means at a rst rate, second means to rotate said radiating means about a second axis parallel from said antenna axis at a second rate, said first rotation being v simultaneous with said second rotation.

2. A radio navigation systemfor reduction of errors due to reflecting obstacles comprising means for radiating energy in a directive pattern, means for applying energy of a radio frequency carrier to said radiating means, first means associated with said radiating means to effect rotation of said pattern about a first axis at a rstrate, second means to rotate said radiating means about a second axis parallel to said first axis at a second rate, said first rotation being simultaneous with said second rotation and a receiver to receive and detect signals from said radiated energy determining the position of said receiver relative to said radiating means.

3. A radio navigation system for'reduction of errors due to refiecting obstacles comprising means for radiating energy in a directive pattern, means for applying energy of a radio frequency carrier to said radiating means, first means associated with said radiating means to effect rotation of said pattern about alfirst axis at a first rate, second means to rotate said pattern about a second axis displaced from said first axis at a second rate, said first rotation being coincident with said second rotation, a receiver to receive and detect signals from said radiated energy determining the position of said receiver relative to said radiating means, and filter means coupled to said receiver References Cited in the le of this patent UNITED STATES PATENTS 2,377,902 Relson June 12, 1945 2,490,050 Hansel Dec. 6, 1949 2,511,030 Woodward lune 13, 1950 2,535,850 Hammond Dec. 26, 1950 2,547,066 Wagner Apr. 3, 1951 

